Hitherto, an air bag device mounted to an automotive steering wheel has been widely used. The air bag device comprises a flat bag-shaped air bag, a cover member that covers the air bag and an inflator that feeds the air bag with a gas. Usually, with the aid of a given folding method, the air bag is kept folded small in the cover member. Upon sensing a shock of collision by a sensor, the inflator ejects a gas to expand the air bag and breaks the cover member with the pressure of the air bag expansion thereby projecting the expanded air bag from the broken cover member and placing the expanded air bag in front of a passenger to absorb the shock applied to the passenger.
Nowadays, methods of folding an air bag that assure improved expansion characteristic, automatic production and reduction in production cost have been seriously pursued.
For example, as a method of folding an air bag for a driver that shows a circular shape when spread flat, a method is known which comprises forming narrow portions of the air bag with the aid of four sword-like tools, and then shrinking the air bag by using a slider for folding the air bag disorderly (see for example Patent Reference 1). In this method, the narrow portions of air bag that are shrunken by the slider are folded without proper mating and thus, upon expansion of the air bag, the narrow portions are suppressed from mutual interference, which is an advantageous effect. However, since the portions that are shrunken by the slider are folded disorderly from the radially outer side, the inflation gas fed from a center portion is not smoothly led to a peripheral portion of the air bag, which makes it difficult to quickly expand the peripheral portion of the air bag.
Another method is also known which comprises shrinking the air bag to its inside by using a plurality of plate like sliders (see for example Patent Reference 2). However, in this method, like in the above-mentioned Patent Reference 1, the gas fed from the center portion fails to smoothly and quickly reach the peripheral portion of the air bag. Furthermore, due to usage of the plurality of sliders, the construction is complicated and the production cost is increased.
Still another method is known which comprises after folding the air bag like an accordion, bending the folded air bag (see for example Patent Reference 3). However, in this reference, an automatic folding method suitable for the accordion like folding and the bending method for the folded air bag are not well described. Furthermore, when the air bag is folded like an accordion, that is, when the air bag is so folded that throughout entire construction of the air bag, a plurality of portions of the air bag are folded in one direction along respective parallel lines, it occurs that the air bag shows a high tendency in expanding direction when expanded, and thus, it is difficult to provide the air bag with a desired expanding characteristic.
In view of the above, a method has been proposed which comprises providing the air bag with radially projecting ear-shaped margin portions, gathering portions between the margin portions at a center portion of the air bag, turning the air bag while folding the same and putting the margin portions along the gathered portions (see for example Patent Reference 4 and Patent Reference 5). In this method, the gas is smoothly supplied to a peripheral portion of the air bag along the margin portions, and thus, smooth and quick expansion to the peripheral portion is assuredly achieved. However, in this method, for turning the air bag that is under the folding process, it is necessary to provide a production device and thus production cost is increased.
Patent Reference 1: Tokuhyo 2000-501354 (FIGS. 9 to 11)
Patent Reference 2: Tokkaihei 10-129381 (FIGS. 3 and 4)
Patent Reference 3: Tokkyo 3579702 (FIGS. 1, 2 and 5)
Patent Reference 4: Kokusai Koukai 97/35745 pamphlet (FIG. 44)
Patent Reference 5: Kokusai Koukai 2001/076917 pamphlet (FIGS. 9, 10 and 13)